The present invention relates to improvements in filters and more particularly to rotary vacuum filters of the type utilized in the pulp and paper industry for filtering pulp slurries.
Rotary vacuum filters are well known and utilize filtration members which comprise either a rotatable cylindrical drum or spaced cylindrical discs mounted for rotation in a tank adapted to receive a feed slurry. Filter media is provided about the outer surface of the filtration members and as the latter are rotated a sub-atmospheric pressure vacuum is created in portions thereof through a period of the filtration cycle as the members are rotated through the slurry to draw filtrate from the slurry into the filtration members and form a filter cake on the surface of the filter media. A valve assembly is provided to cut off the vacuum during other periods of the filtration cycle when the cake bearing portion is clear of the tanks whereat the filter cake is removed from the filter media by any well known doctoring means. The supply and cut off of the sub-atmospheric pressure vacuum is controlled by a valve assembly which is located concentrically with the center of the filtration members at an end trunnion and through which valve assembly the filtrate is withdrawn outwardly of the filter.
The sub-atmospheric pressure necessary to achieve the mentioned desired vacuum in filters of this type is often created by use of a barometric leg and the present invention is directed to improvements in the performance characteristics thereof. Barometric legs or drop legs, as they are also called, comprise elongated pipes which depend from the filtrate control valve assembly to a filtrate tank located at a position below the filter with the free end of the leg submerged in a seal pit located in the filtrate tank. In some instances the filter may be located on one floor of the operating plant while the drop leg extends from the valve assembly to the floor below in order to achieve a leg of sufficient length whereby the velocity of the air-liquid exiting from the valve assembly and moving down the leg is sufficient to sustain a vacuum in the filtration members during the filtration cycle. The barometric leg offers a simple energy conserving means for draining filtrate from a filter since it functions as both an efficient filtrate pump and air compressor with the input power supplied by the liquid drop in the leg, the vertical length of which varies as is necessary for effective operation.
In use as the filtrate and air mixture drawn from the filter exits from the valve assembly to the drop leg, heat is produced due to friction and turbulence created by the pipe walls of the leg and fittings. The velocity of the air liquid mixture varies with the pressure within the leg and becomes less as the air is compressed. The kinetic energy possessed by the air liquid mixture is converted to heat upon exit from the drop leg into the seal pit causing turbulence in the liquid therein. It has been found that the turbulence created within the seal pit results in a noticable loss of energy in the filtration system and a consequent reduction of operating vacuum in the filter.
It is an object of the present invention to provide an improved barometric leg assembly for a rotary vacuum filter.
It is a further object to provide a novel barometric leg which reduces the velocity head of the air-liquid flow exiting from the leg to correspondingly reduce turbulence in the seal pit resulting in a pressure gain and an increased vacuum in operation of the filter.
Another object is to provide novel means for shortening the length of the barometric leg without a corresponding loss in efficiency of operation thereof.
A still further object is to provide a novel barometric leg having a drop velocity high enough for air entrainment and with an exit velocity from the leg low enough to prevent wasted power in turbulence within the sealed pit.